Utilization of the piezoelectric effect in different devices is known, and one can mention very different applications in which these devices are presently effectively used, including process control, nondestructive testing, intrusion detection, measurement of elastic properties, medical diagnosis, delay lines, and signal processing, etc.
These and many other applications of the piezoelectric effect are well described in the literature and can be found, for example, in the monograph, "Ultrasonics, Fundamentals, Technology, Applications," by Dale Ensminger, Marcel Dekker, Inc., 1988.
The most important part of such devices is the piezoelectric transducer, containing crystalline material, which, being exposed to a pressure applied along certain crystallographical axes, is able to produce electrical charges on preferred crystallographic surfaces, or, on the contrary, being exposed to a voltage applied between two preferred surfaces of the crystal, is able to produce a stress or strain along its axes.
Depending on the amount of electrical energy convened into mechanical energy of acoustical waves or vice versa, a particular transducer can be used in devices for low- or high-intensity applications.
Low-intensity applications are usually those wherein the primary purpose is to transmit the converted energy as acoustic waves through a medium without changing its state and to measure the propagation characteristics for obtaining engineering data for materials.
Devices based on piezoelectric transducers which operate on low-intensity principles include, for example, height detectors, liquid-level gauges, devices for measuring of amount of solids in suspension, and so on.
High intensity applications are those which produce changes in or effect the medium, or the contents of the medium, through which the converted energy propagates.
There are various mechanisms which promote this effect. Some of them can be related to heat, chemical effect, cavitation, mechanical effects, etc.
High-intensity applications of piezoelectric transducers are numerous, and for only a few examples of leading commercial high-intensity applications one can mention ultrasonic cleaning tanks, ultrasonic machining tools, ultrasonic welders, atomizers for oil burners, dewatering devices, and so on.
In all the above mentioned applications there takes place conversion of the electric energy into mechanical energy of acoustical waves or vice versa, not for the purpose of generating energy, but for achieving a particular mechanical, chemical or other effect, in other words, these devices do not serve as energy generators, per se.
There are also publications in which there is mention of the utilization of piezoelectric transducers in generators, for example, those driven by combustion engines, (see U.S. Pat. No. 4,511,818). This known device comprises a combustion engine which has piston driven by explosion of a fuel mixture and a piezoelectric transducer responsible for generating voltage and applying this voltage for return piston movement.
Despite the fact that this device includes piezoelectric transducer and combustion engine, it cannot be considered a piezoelectric generator of energy per se, since that the voltage generated in the transducer serves not for energy output, but for an auxiliary purpose; namely, for the return movement of a second piston of the combustion engine.
Another device, known in the art as V. M. Falkov's generator and described in the Soviet invention certificate, SU 613,421, is dedicated to converting the energy of flowing gas into electric energy.
This generator uses piezoelectric elements to generate an electric signal when the elements are exposed to a flow of gas or steam entering the case where these elements are mounted. The case with piezoelectric elements is installed in the vehicle, and when it moves on a road or in the air, gas or steams cause deformation and oscillation of the piezoelectric elements thus generating an electric signal collected by current collectors.
The main disadvantage of this and other known generators comprising combustion chambers and utilizing the piezoelectric effect for converting of mechanical or heat energy into electric energy (or vice versa) is their low efficiency, i.e., low value of the ratio of the power out to the total power into the system.
This disadvantage is intrinsic to generators in which the transfer of acoustic energy takes place inside the combustion chamber and is associated with the fact that it is very difficult to ensure both high efficiency of transformation of heat energy from the exploding fuel into acoustic energy (in the combustion chamber) and at the same time high efficiency of transformation of acoustic energy into electric energy (in the transducer). To ensure both of the above conditions, the combustion chamber should be as small as possible; however, this condition makes difficult effective thermal insulation and reliable functioning of the transducer. Besides, a small combustion chamber requires more frequent cleaning because it becomes dirty more often.
As a consequence, there should always be a compromise between the ultimate dimensions of the chamber while still ensuring its reliable functioning and achievable efficiency of the generator.